1. Field of the Invention
The present invention relates to a liquid crystal display device used as display means of a computer or the like, and a method for inspecting the same.
2. Description of the Related Art
A liquid crystal display device includes a glass substrate and a counter glass substrate facing the glass substrate, and the glass substrate and the counter substrate are attached to each other with a liquid crystal material being introduced therebetween. By applying an electric signal between respective electrodes of the glass substrate and the counter electrode, incident light from an external light source is modulated, whereby information is displayed on the screen.
Such a liquid crystal display device has advantages over a CRT (cathode-ray tube) such as low power consumption, small thickness and light weight. Therefore, the liquid crystal display device has attracted attention as a next-generation display device, the production quantity thereof has been significantly increased. The liquid crystal display device includes a number of pixel electrodes which are arranged in a two-dimensional array. In order to improve the display quality of the liquid crystal display device, an active-matrix liquid crystal display device wherein switching elements such as thin film transistors (TFTs) are provided for the pixel electrodes have been increasingly produced.
However, due to the complex production process of the switching elements, a defective display may be caused by scanning lines (i.e., gate lines) and signal lines (i.e., source lines), line defects resulting from a short-circuit, point defects resulting from defective switching elements, non-uniform display or the like. Since the yield of the liquid crystal display devices is not 100%, sufficient inspection of the display quality must be conducted. Moreover, in order to improve the yield and display quality of the final liquid crystal display devices, the inspection result must be rapidly fed back to the production process. Therefore, it is important to conduct array inspection and/or operating-display inspection and analyze the inspection result. The term "array inspection" as used herein indicates inspecting the liquid crystal display device at the time the switching elements such as TFTs are completed. The term "operating-display inspection" as used herein indicates inspecting the liquid crystal display device before mounting expensive elements such as driver circuits, and TAB (Tape Automated Bonding), that is, inspecting the liquid crystal display panel by starting the display.
FIGS. 8A, 8B and 8C show a general liquid crystal display panel, wherein FIG. 8A is a plan view of the liquid crystal panel, FIG. 8B is a front view thereof, and FIG. 8C is a side view as viewed from the right side of the liquid crystal display panel.
The liquid crystal panel shown in FIGS. 8A, 8B and 8C includes a glass substrate (array substrate) 114 and a counter glass substrate 113 which faces the glass substrate 114, the glass substrate 114 and the counter glass substrate 113 being attached to each other with a liquid crystal material introduced therebetween. The glass substrate 114 includes pixel electrodes, active elements (i.e., switching elements), gate-bus lines, source-bus lines (not shown) and the like. In the outer periphery of the glass substrate 114, source-signal input terminals 110 and gate-signal input terminals 111 are provided as starting-signal input terminals. In the liquid crystal panel, image information is displayed on a display region 112.
A liquid crystal display device is mostly used in the products such as notebook personal computers. In such products, reduction in the total area of the elements in the outer periphery (i.e., peripheral region adjacent to the display region) of the liquid crystal panel has been desired in order to increase the display region 112 of the liquid crystal panel. Therefore, a method for inputting a signal from two sides of the panel, as shown in FIGS. 8A, 8B and 8C, is widely used. Moreover, the pitch of the input signal terminals 110 and 111 has been increasingly reduced in order to improve resolution.
The liquid crystal display panel having such a structure as described above is generally inspected as follows: a probe pin is put into contact with each of a multiplicity of signal input terminals of the liquid crystal display panel. Then, an inspection electric signal is applied thereto, whereby the liquid crystal panel is started. In this state, the liquid crystal panel is inspected and/or analyzed for defects with its operating patterns being changed. The inspection and/or analysis is conducted by visual recognition and/or by using a camera.
Such a conventional liquid crystal display device has the circuitry as shown in FIG. 9.
Source lines 101 and gate lines 102 are arranged in a matrix so as to cross each other with an insulating film (not shown) interposed therebetween. The signal input terminals 110 are provided on the respective extensions of the lines 101, and the signal input terminals 111 are provided on the respective extensions of the lines 102. The portions of the source and the gate lines 101, 102 in the peripheral region are referred to as the extensions of the respective lines 101 and 102.
In the case of the array inspection, a probe (not shown) is made in contact with each of the signal input terminals 110 and 111, and a signal voltage is applied thereto. When the liquid crystal panel is started as the liquid crystal display device, TAB or the like is bonded to the signal input terminals 110 and 111.
A TFT 105 is provided at every intersection of the source lines 101 and the gate lines 102, and the drain electrode of each TFT 105 applies a signal to a corresponding pixel electrode 103. Each pixel electrode 103 faces a corresponding counter electrode 107 with a liquid crystal layer 106 interposed therebetween, and holds a display data voltage in the liquid crystal layer 106, thereby displaying an image on the screen. Since the source lines 101 and the gate lines 102 are electrically insulated from each other by the insulating film, defects may be produced in the display due to static electricity. For example, the static electricity may be generated during the production process of the TFTs, the production and packaging processes of the liquid crystal panel and the like. In such a case, the static electricity thus generated charges the source lines 101 and/or the gate lines 102, whereby a voltage which is much higher than the actual driving voltage may be applied, causing dielectric breakdown of the insulating film and/or undesirable characteristics of the switching elements. In order to prevent such electrostatic damage, a short ring 108 for electrically short-circuiting the lines 101 and 102 is provided along the outer periphery of the liquid crystal display device. It should be noted that the two-dotted line 109 shown inside the short ring 108 represents a separation line.
Another structure as shown in FIG. 10 has also been proposed. This structure is disclosed in Japanese Laid-open Publication Nos. 63-220289 and 63-10558. FIG. 10 is a schematic enlarged view of a signal input terminal portion of a typical liquid crystal display device disclosed in Japanese Laid-open Publication Nos. 63-220289 and 63-10558.
Referring to FIG. 10, the liquid crystal display device includes an array substrate 115 and a counter substrate 116 which faces the array substrate 115, the array substrate 115 and the counter substrate 116 being attached to each other with a liquid crystal material being introduced therebetween. The array substrate includes display switching elements (not shown) such as TFTs. In the liquid crystal display device, image information is displayed on a display region 117. Source lines 118 and gate lines 119 are provided on the array substrate 115 as required for image display. Signal input terminals 120 are provided on the respective source lines 118, and signal input terminals 121 are provided on the respective gate lines 119. In order to prevent the above-mentioned electrostatic damage, a short ring 122 is provided in the outer periphery of the liquid crystal display device, and each of the lines 118 and 119 are electrically connected to the short ring 122 through 2-terminal operating a-Si (amorphous silicon) TFTs 123a and 123b or the like. The TFTs 123a and 123b are provided as diodes of opposite directions.
In the structure shown in FIG. 9, the short ring 108 is provided along the outer periphery of the liquid crystal display device and the lines 101 and 102 are directly electrically connected to the short ring 108. In this case, electrostatic damage can be prevented during the production process such as pattern formation. During the processes such as display inspection process and assembling process, however, a signal voltage must be individually input to each terminal. Therefore, it is necessary to cut off the short ring 108 by the separation line 109. Accordingly, the electrostatic damage cannot be prevented.
In the structure shown in FIG. 10, the a-Si TFTs 123a and 123b each serving as a protection element are provided between the short ring 122 and each of the lines 118 and 119. In this case, when the resistance of the protection elements is low, slight leakage occurs between the lines 118 and 119, causing disadvantages in the array inspection and the operating-display inspection. On the other hand, when the resistance of the protection elements is high, the electrostatic damage cannot be completely prevented. Accordingly, in the structure of FIG. 10, it is very difficult to optimize the resistance of the protection elements.